Thermal characterizations and modeling have been carried out on a 0.15 μm× (4×50) μm gate GaN-/SiC-based high-electron-mobility transistor varying the temperature from -40 °C to 150 °C and the frequency up to 50 GHz using on-wafer measurements. The thermal behavior of the dc parameters along with thermal resistance estimation and the equivalent circuit parameters along with cutoff and maximum frequencies were analyzed and reported using a single device. The temperature coefficients of these parameters were presented and deduced the influence of thermal effects on device parameters. These results are important for the circuit designer for future advancement and design optimizations of GaN-based monolithic microwave ICs.